The other day I went down to the little shop in the lobby of our building for a snack. I couldn't decide whether I wanted regular M&M's or Peanut Butter M&M's so I bought them both. On the way back upstairs to the office, I noticed something strange on the labels. Each had cost $1, but the pack of Peanut Butter M&M's was a very tiny bit lighter: 0.06 ounces lighter!

I wanted to know why, so I called a couple of experts and asked for their theories:

Theory No. 1: Peanut Butter M&M's are more expensive to make.

Bill Edwards used to work for one of Mars' big rivals, a British company called Rowntree's that developed a candy similar to the M&M's called Smarties.

Edwards spent a lot of time reverse engineering his competitors' products. And he had a theory that could explain the lighter package. It's possible, he said, that Peanut Butter M&M's are more expensive to make, so they put fewer in the package.

"You are not going to do wonders for your business if you bring out a new product that is less profitable than your existing ones," Edwards said.

But Edwards then quickly ruled this out. Peanut butter, he says, is generally less expensive than chocolate, so you would expect to get more in a Peanut Butter M&M's package than in a package of regular, Milk Chocolate M&M's, if cost was the reason.

Theory No. 2: Each bag gets filled by a machine that drops a precise number of M&M's in each package. The Peanut Butter M&M's weigh less than original M&M's, so the peanut butter package is lighter.

Sanat Kumar, chair of the chemical engineering department at Columbia University, had this idea. He and I took out his superprecise scales and emptied out a couple of packages of M&M's. We started counting how many M&M's were in each peanut butter M&M's package. Were they the same? Nope: One had 28 M&M's, but the next had 26. Another beautiful theory destroyed by data.

But weighing the M&M's did help us discover something weird that would turn out to be a critical clue. It was something I'd never noticed while I was stuffing these things in my mouth — Peanut Butter M&M's and Milk Chocolate M&M's are totally different species. The regular M&M's are small, very precise candies with very precise weights, but the peanut butter ones are larger, lumpier and more irregular. When we put them on Kumar's scale, the Peanut Butter M&M's were much heavier, and their weights were all over the place.

And this larger size turned out to be the key to the whole puzzle.

Mars, the company that makes M&M's, declined our requests for an interview. But it provided a statement.

The answer apparently has nothing to do with the price of peanuts compared to chocolate. Or the number of M&M's in a bag. It is a different number entirely that matters: calories.

Here's the statement:

"Mars Chocolate North America is committed to health and nutrition, which includes efforts to manage portions and calories, so all of our products are 250 calories or less per portion. This means that weights may vary from pack to pack, and are always clearly labeled on the product package."

Sanat Kumar and I did the math, and their calorie explanation made sense. The Peanut Butter M&M's are so big, adding just one more M&M to one of the packages would push the calorie count over 250.

Mystery solved! Except for one thing. Why exactly were the peanut butter M&M's so much larger? Did it have to do with how they were made?

I sent a follow-up email to Mars and got a one-sentence response.

"Because of proprietary reasons, we are unable to elaborate on the size of Peanut Butter M&M's."

Copyright 2014 NPR. To see more, visit http://www.npr.org/.

Transcript

STEVE INSKEEP, HOST:

You know, if you actually read the labels on food you eat you'll find some curious things. David Kestenbaum of our Planet Money Team recently found himself staring a pack of M&Ms and he saw one number of particular that seem odd.

DAVID KESTENBAUM, BYLINE: I had gone down to the lobby of our building here to buy a snack and I couldn't decide which kind of M&M to get. So I bought two packs - one regular M&M, one peanut butter filled M&M. In the elevator back up to the office I found myself reading the packaging and I noticed this strange thing. Here I have one - a pack of original M&M weighs 1.69 ounces. The peanut butter ones - slightly less 1.63 ounces. They were the same price, a dollar each, but you get slightly less of the peanut butter M&Ms, 0.06 ounces less - it's a tiny difference way down at the third decimal place. I wanted to know why so I reached out to Mars, the company that makes M&Ms, and while I was waiting to hear back I made some other calls and I discovered this whole weird world.

BILL EDWARDS: You should appreciate, actually, that the confectionery industry is incredibly secretive.

KESTENBAUM: This is Bill Edwards, he used to work for of Mars' big rivals - a company called Rowntrees that makes a British candy called Smarties that are very similar to M&Ms.

EDWARDS: I mean, more secrets have escaped the places where they make nuclear weapons than have ever escape the places from the major confectionery firms.

KESTENBAUM: Edwards spent a lot of time reverse engineering his competitors products and he had a theory here. Maybe, he said, peanut butter M&Ms are more expensive to make so they put fewer in the package. As a way to keep the profit margin the same.

EDWARDS: You're not going to do wonders for your business if you bring out a new product that is less profitable than the existing ones.

KESTENBAUM: Oh, so like if you had M&Ms that were out there - there's a certain profit margin - you're going to introduce peanut butter M&Ms. You would never want to introduce that where you're going to make less money on each pack.

EDWARDS: No, no.

KESTENBAUM: But then having proposed this beautiful theory, he basically rolled it out. peanut butter, he says, is actually less expensive. So if anything, he'd expect you to get more in a peanut butter M&M package. So much for the economic explanation, maybe there was a scientific one. Is it okay if I buy a lot of M&Ms? I went back down to the little store the lobby and I bought what scientists would call a statistically significant sample size. 10 of these 10 of these. I took the M&Ms up to Columbia University to a guy who had a really, really precise scale, Sanat Kumar. He's chair of the Chemical Engineering Department.

SANAT KUMAR: This is a tenth of a gram, and is going to a tenth of a milligram. So a thousand times smaller than what you are looking for.

KESTENBAUM: Sanat had a different theory. Maybe, he said, each bag gets filled by a machine that drops a precise number of M&Ms in each bag. If the peanut butter ones were just a little bit lighter - that could explain why the package came out lighter. So we started counting.

KUMAR: 10 12 14 16 18.

KESTENBAUM: One bag had 28. Another had 26.

KESTENBAUM: Ok, 27 28 - so they are not keeping the number constant.

KUMAR: It's all over the map. This is truly amazing. I have no idea what they're doing.

KESTENBAUM: Another beautiful theory destroyed by data. We did discover something weird though - something that somehow I had never realized what I was eating these things. The peanut butter M&Ms and Ms and the regular M&Ms are like totally different species. The original chocolate M&Ms are these small very precise candies with precise weights. The peanut butter M&Ms, by comparison, are monsters - they're larger, they're lumper and more irregular. As Sanat puts it, the weights had a larger standard deviation. We measured on the fancy scale.

KUMAR: 1.62, 2.0 - fluctuation is amazing.

KESTENBAUM: The larger size turns out to be key to the whole puzzle. I got an email back from the Mars company declining our interview request but including a statement which explained that for health and nutrition reasons all Mars products are designed to be 250 calories or less per person. Calories - we did the math. It turns out because the peanut butter filled ones are so big if you add just one more of them to these little packages you would push the calorie count over 250. That, it seems, is why the package weighs a little less. Nothing to do with the price of peanuts compared to Cocoa - or with the machine that fills the little bags. Calories. I sent a follow-up email asking why the peanut butter M&Ms were so much larger. I got back a one sentence response, quote, "because of proprietary reasons we are unable to elaborate on the size of peanut butter M&Ms". David Kestenbaum, NPR news.